Linux-kernel mailing list archive 1999-37,: Re: difference b/w 32-bit vs. 64-bit kernel

# Re: difference b/w 32-bit vs. 64-bit kernel

Avenger (asmlinkage@softhome.net)
Mon, 13 Sep 1999 23:02:39 +051800

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On Mon, Sep 13, 1999 at 06:55:57PM +0530, Sandeep Kumar wrote:
> can anyone elaborately tell the differences b/w
>
> 1. 32-bit OS vs. 64-bit OS
> 2. 32-bit Kernel vs. 64-bit kernel
>
> is it necessary to have 64-bit I/O, 64-bit addressing etc. (i.e. all the 64-bit
> things to be there for an OS to be called 64-bit?).
Hi there,
A recent thread that was on the Linux Kernel mailing List might be of help
On Fri, 10 Sep 1999 11:06:53 +0530, Sachin Tilloo wrote:
> I have heard about 16/32 bit OS. As far as i know a 32 bit Os means
> that the instructions that are used to code OS knew only 32 bit
> operations. Am i right !!!

I general it means that it has a 64 bit address space. You can always do
non-32 bit operations on a 32 bit platform: load a single byte is one of
them (of course given that the CPU can do it).

> Now if i have say a 32 bit Os and the length of my data bus of PC is
> 64 bit, can i not run a 64 bit program on my PC.

The length of the data bus is independent of the CPU. For instance: the
80386SX processor was a 32 bit processor with a 16 bit databus so it could
be used with the cheaper 80286 compatible support chips. This was of
course slower because a single 32 bit load or store had to be divided into
two 16 bit load/stores.

> I mean if my compiler and CPU can undertand that instruction where
> does the OS come in between to prevent it.

The OS doesn't prevent you to do it. Unless... the OS only saves 32 of the
64 bits in a register during a context switch. No problem when you run
a single process using 64 bit registers, but as soon as you use two, they
will most likely be messed up.

> I think as far as i am not needing dynamic memory support from OS
> i can run say a simple program which transfers 64 bits from
> memory location to my 64 bit register.

As long as you use only one of those programs.

Hope this helps

Regards

Syed Khader Vali | The road to wisdom? |
sidcarter@usa.net | Well it's plain and simple to express:|
http://linuxindia.virtualave.net | Err and err and err again |
http://sidcarter.8m.com | but less and less and less. |

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